Description of multilayered gamma-ray exposure buildup factors up to 40 mfp by the approximating model

An approximating formula recently proposed by the authors for gamma-ray bui ldup factors of multilayered shields was applied for very thick shields up to 40 mfp. For this purpose, modifications were made to the model and the f itting method to improve the data reproducibility. The previous model was e xpanded so that it included both the plane-normal and point isotropic geome tries. The verification test of the modified model was made for three materials; w ater, iron and lead. The separately published data of double-layered shield s for point isotropic buildup factors calculated by EGS4 from 0.1 MeV to 10 MeV were used as well as newly calculated data at 1 MeV for the plane-norm al geometry, and data for the point isotropic geometry of triple-layered sh ields at 1 and 10 MeV. The present formula generally shows a very good reproducibility of the mult ilayer buildup factors, even in case of very thick shielding problems. The observed error between the approximating description and the EGS4 data is 1 5% in the intermediate energy range, about 30% in the higher energy range, and 35% at 0.3 MeV. However, the error in the approximation reaches a facto r of 4 in the worst case at 0.1 MeV.